builtins + test helpers: use return instead of exit() in cmd_*

Change various cmd_* functions that claim to return an "int" to use
"return" instead of exit() to indicate an exit code. These were not
marked with NORETURN, and by directly exit()-ing we'll skip the
cleanup git.c would otherwise do (e.g. closing fd's, erroring if we
can't). See run_builtin() in git.c.

In the case of shell.c and sh-i18n--envsubst.c this was the result of
an incomplete migration to using a cmd_main() in 3f2e2297b9 (add an
extra level of indirection to main(), 2016-07-01).

This was spotted by SunCC 12.5 on Solaris 10 (gcc210 on the gccfarm).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sh-i18n--envsubst.c

@@ -104,12 +104,12 @@ cmd_main (int argc, const char *argv[])
   if (ferror (stderr) || fflush (stderr))
     {
       fclose (stderr);
-      exit (EXIT_FAILURE);
+      return (EXIT_FAILURE);
     }
   if (fclose (stderr) && errno != EBADF)
-    exit (EXIT_FAILURE);
+    return (EXIT_FAILURE);
 
-  exit (EXIT_SUCCESS);
+  return (EXIT_SUCCESS);
 }
 
 /* Parse the string and invoke the callback each time a $VARIABLE or